Pickling inhibitor



Patented Aug. 4, 1936 UNITED STATES PATENT OFFICE PICKLIN G INHIBITOR No Drawing. Application December 29, 1933, Serial No. 704,475

16 Claims.

This invention relates to the art of pickling ferrous metals. More particularly, it relates to the use as corrosion inhibitors in ferrous metal pickling baths of a class of materials which have been found to be extremely effective in retarding the attack of pickling acid on the metal or metals being pickled, but at the same time not to impair to any substantial extent the action of the acid on scale and rust. The compounds to which the invention relates, while not such as to dissolve readily in all of the more common acids, have the advantage that, once having been dispersed in the manner hereinafter described, they remain in dispersion in the acids ordinarily used for pickling and hence do not form a scum on the surface thereof.

It has been customary heretofore to remove scale and rust from the surface of iron, steel and other ferrous metals by pickling the metal in a dilute solution of sulphuric or hydrochloric acid, usually heated to temperatures of approximately Ill-90 degrees C. to accelerate dissolution of the iron oxides. This procedure involves the disadvantage that the pickling acid, in addition to dissolving the iron oxides, attacks the ferrous metal itself, liberating hydrogen gas which, on escaping to the atmosphere, carries some of the acid with it in the form of a fine, highly corrosive mist which is very disagreeable to persons in the vicinity. The solution of the ferrous metal in the pickling acid is also undesirable, both because of the loss of acid and because of the fact that the process of solution is not uniform but localized, causing pitting of the ferrous metal surface.

This invention resides, among other things, in the discovery of a class of materials which inhibit the dissolution of the ferrous metal by the pickling acid, but which have little or no retarding effect on the action of the pickling acid on scale and rust. The materials in question are reaction products of formaldehyde and an ammonium sulphide such, for example, as ammonium hydrogen sulphide, NHd-lS; ammonium sulphide, (NHOaS; and ammonium thio cyanate, NH4CNS. The exact constitution of these reaction products is not definitely known; consequently, they will be referred to herein as reaction products. It will be understood that it is not intended thereby to restrict the compounds in question to products made by the interaction of formaldehyde and an ammonium sulphide, for it is at least possible that a given compound falling within the scope of the invention may be 55 prepared by more than one method.

Example 1 The reaction products of the invention are very easily prepared by reacting formaldehyde with the desired ammonium sulphide. The reac- 5 tion is exothermic and it is therefore generally preferable to react the materials in the cold. One method which has been found particularly suitable is that in which 1000 parts by weight of an approximately 40% solution of ammonium Wk, cooled with approximately parts of ice, are mixed with 1620 parts by weight of an aqueous 37% fognaldehyde solution. An exothermic reaction takes" place with the separation of a white gummy mass which 15 gradually r es i niij1es. This material, after being permitted to stand for some time, is ground to a fine powder, washed with water, and dried.

In the above example, the formaldehyde and ammonium hydrogen sulphide are reacted in the proportion of five mols of formaldehyde to two 20 mols of the sulphide. Although its exact constitution is not known, this particular reaction prod- 5 uct has been referred to in the literature as penta methylene diamino disulphide. The following equation has been suggested for its forma- 25 tion:

It will be understood that despite the fact that a name and formula have been assigned to this reaction product, there is doubt as to its constitution.

While penta methylene diamino disulphide has been found to be an extremely effective pickling inhibitor, it is to be understood that formaldehyde and ammonium hydrogen sulphide may be reacted in substantially any other proportions. Likewise, other ammonium sulphides such as ammonium sulphide, (NI-I028, and ammonium thio cyanate, NH4CNS, may be employed in place of, or in admixture with, the ammonium hydrogen sulphide. Results comparable to those obtained with penta methylene diamino disulphide" are obtained in such cases.

Example 2 Illustrating the employment of ammonium thio cyanate, the reaction product of formaldehyde and ammonium thio cyanate is prepared by dissolving 152 grams of ammonium thio cyanate in 300 grams of water and adding thereto 180 grams of an aqueous 37% formaldehyde solution. After an induction period of 30 minutes, the reaction begins. An amorphous, light colored reaction product, which upon being heated for 15 minutes in boiling water becomes crystalline, forms. The dried material melts at 173-177 degrees C.

Example 3 The reaction product of ammonium thio cyanate and formaldehyde may also be prepared by adding a solution of 820 grams of 37% aqueous formaldehyde to a solution of 300 grams of ammonium thio cyanate in 600 grams of water and permitting the mixture to stand for a period of 16 hours. A heavy solid cake forms during the period of standing. Thereafter the mixture is heated to boiling for a period of approximately 15 minutes, after which it is cooled and the resulting solid filtered and dried. The product is obtained in a yield of 306 grams and melts at 195-200 degrees C.

Example 4 Illustrating the employment of ammonium sulphide UNI-102s], 160 grams of an aqueous 37% formaldehyde solution are added to 200 grams of a 20-21% ammonium sulphide solution [(NH4) 28] in water cooled with ice, the ammonium sulphide being stirred during the addition of the formaldehyde. The reaction is exothermic, the temperature rising to 40 degrees C. during the addition of the formaldehyde solution. A sticky product forms almost at once. On standing overnight, it solidifies to a white mass. After being vacuum dried at a temperature ranging from 50 to degrees C. for 10 hours, it is obtained in a yield of 45 grams.

Illustrating the effectiveness of the reaction products of this invention, mention may be made of comparative tests on two pickling baths, each consisting of 800 cc. of a 6% aqueous solution of sulphuric acid heated to 180 degrees F. One bath contained 0.059 gram of penta methylene diamino disulphide, while the other contained no inhibitor. Thin sheets of clean, hot rolled steel 3"x4" adapted for tin plating were weighed and then placed in each bath for 40 minutes, the temperature being maintained at approximately 180 degrees C. After this period, the samples were dried and reweighed. Those which had been pickled in the bath containing no inhibitor had sufiered a loss of weight averaging 0.0455 grams per square inch, while those taken from the bath containing the penta methylene diamino disulphide had lost an average of only 0.0133 gram per square inch.

Further illustrative of the efiectiveness of the reaction products of this invention are the hereinafter described tests conducted upon sheets of hot rolled steel 3"x4x1/64" adapted for tin plating. These sheets were immersed in pickling baths, each consisting of approximately 1000 cc. of a 6% aqueous solution of sulphuric acid heated to a temperature of 180 degrees F. One of the baths contained no inhibitor, one contained 0.06 gram of the reaction product of ammonium sulphide and formaldehyde prepared as described in Example 4, and the other contained 0.06 gram of the reaction product of ammonium thio cyanate and formaldehyde prepared according to Example 3. The sheets were weighed before being placed in the bath and after a period of 20 minutes were removed, dried and reweighed in order to obtain the weight of the sheets minus any scale which may have been present. They were then returned to the respective pickling baths and after a period of an hour were removed, dried and again weighed.

In the practice of this invention on a commercial scale, an acid solution containing sulphuric acid is prepared in the usual manner and a small proportion of the inhibitor is dissolved therein; for example, one pound of the inhibitor to from to 1000 pounds of acid. This is done, preferably, by first dissolving the inhibitor in concentrated sulphuric acid and then diluting this solution with water to the desired concentration. The metal, which is preferably first freed of grease by treatment with an organic solvent or with an alkali, is treated in this bath in the customary manner except that it may be left in the bath a longer time than ordinarily feasible since there is little danger of over-pickling.

Metals may be pickled in the improved pickling solutions containing the inhibitors of the class hereinabove described in many widely different manners. Thus the metal may, if desired, be completely immersed in the acid, being carried into and out of the solution by known automatic devices. Alternatively, the solution may be sprayed onto the surface of the metal and later washed off by a water spray. The concentration of the acid and the temperature of the solution may be varied as desired and the inhibitors may be added in either greater or less proportions than described in the examples above. Even proportions as low as 0.01% by weight of the inhibitor based on the amount of acid employed are effective. Comparatively large proportions may be necessary in some cases to reduce the corrosion of the metal to a minimum.

Although drochloric a c i d and sulphuric acid are generally employed in pickling operations because of their low cost, any of the other nonoxidizing acids may be substituted therefor in whole or in part. For example, ickling solutions containing phgghoric or hydro uoric' acid are usefiflijr'rliiiany cases. me concentrations ordi nafily'employed in commercial use; namely from six to twelve percent, will generally be preferred.

The reaction products to which the invention relates are, in general, light in color and soluble in concentrated sulphuric acid solutions. Consequently, when employing one of them in a pickling bath, it is generally desirable to first dissolve it in concentrated sulphuric acid and then dilute the solution to the desired concentration or add such solution to the desired bath. This procedure, however, is not necessary in practicing the invention, colloidal dispersions being very effective in many cases. In other cases it is not necessary that the dispersion be colloidal, but it is, of course, generally desirable that the inhibitor be dispersed as well as possible throughout the picking bath.

The property of being efi'ectlvely dispersed in the pickling bath either by colloidal dispersion or by solution makes the reaction products of this invention particularly valuable since it permits the employment of relatively large proportions of the inhibitor without the attendant danger of a layer of scum or dirt being formed on the surface of the pickling bath, which contaminates the freshly pickled metal as it is withdrawn from the pickling solution. The use of a clean acid solution is, of course, desirable in any case but is particularly important where the washing tank is not near the pickling bath. By virtue of these and other advantages, ferrous metals pickled according to the teachings of the present invention are given a superior finish, thus making the invention highly valuable in operations such as tin plating, wherein it is necessary that the iron or steel to be plated be free of spots and stains. The inhibitors of this invention have been found particularly valuable with both low and high carbon steels, either normalized or not normalized. Other advantages are that pitting of the metal is almost completely eliminated, the consumption of acid and of metal is greatly reduced and embrittlement of the metal and formation of acid mist due to the evolution of hydrogen gas are largely prevented.

The reaction products of this invention may be employed as inhibitors by themselves or in diluted form with any of the well-known inhibitor diluents, such as kieselguhr, sulphite pulp and the like. They may of course be employed in the pickling of any metallic surface which lends itself to cleaning by the use of chemical agents in liquid form, but will preferably be employed with ferrous metals.

Although only the preferred embodiments of the invention have been described in detail, it will be apparent to those skilled in the art that the invention is not limited thereto, but on the other hand that various modifications may be made therein without departing from the spirit of the invention.

It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable novelty reside in the invention.

What I claim is:

1. A pickling bath for metals containing a pickling acid and a reaction product of two parts by weight of ammonium hydrogen sulphide and five parts by weight of formaldehyde.

2. A pickling bath for metals containing a pickling acid and a reaction product of formalde-i hyde and an ammonium sulphide.

3. A pickling bath for metals containing a pickling acid and a reaction product of formaldehyde and ammonium hydrogen sulphide.

4. A pickling bath for metals containing a pickling acid and a reaction product of formaldehyde and ammonium sulphide.

5. A pickling bath for iron or steel articles containing sulphuric acid and a reaction product of formaldehyde and ammonium hydrogen sulphide.

6. A pickling bath for iron and steel articles containing a non-oxidizing mineral acid and a small amount of penta methylene diamino disulphide.

7. The process of pickling iron and steel articles which consists in treating said articles in a solution comprising a non-oxidizing mineral acid to which has been added an inhibitor comprising a reaction product of formaldehyde and an ammonium sulphide.

8. The process of cleaning ferrous metals which comprises treating said metals with a non-oxidizing mineral acid containing a small proportion of the reaction product of formaldehyde and an ammonium sulphide selected from the group consisting of ammonium sulphide, ammonium hydrogen sulphide and ammonium thio cyanate.

9. The process which comprises pickling iron or steel in sulphuric acid containing a small amount of the reaction product of formaldehyde and an ammonium sulphide.

10. The process which comprises pickling iron or steel in sulphuric acid containing a small amount of the reaction product of formaldehyde and ammonium hydrogen sulphide.

11. The process which comprises pickling iron or steel in sulphuric acid containing a small amount of penta methylene diamino disulphide.

12. A pickling solution comprising sulphuric acid and a small proportion of penta methylene diamino disulphide.

13. A pickling solution comprising sulphuric acid and a small proportion of a formaldehydeammonium sulphide reaction product.

1'4. A pickling bath for metals containing a non-oxidizing mineral acid and a reaction product of formaldehyde and ammonium thio cyanate.

15. The process which comprises pickling iron or steel in a non-oxidizing mineral acid containing the reaction product of formaldehyde and ammonium thio cyanate.

16. A pickling bath for metals containing a non-oxidizing mineral acid and a reaction product of formaldehyde and an ammonium sulphide selected from the group consisting of ammonium sulphide, ammonium hydrogen sulphide and ammonium thio cyanate.

WILLIAM P. TER HORST. 

